This section is intended to provide a background or context. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
For example, in LTE Rel-12, coverage enhancement for machine type communications (MTC) may potentially extend the LTE coverage by approximately 20 dB beyond the typical range of LTE (such as defined as a cell-edge data rate of 20 kbps). The degree of coverage enhancement varies for different channels based on a link budget analysis. For the (E)PDCCH, the target coverage improvement is 14.6 dB for FDD (2Tx-2Rx) and 19.8 dB for TDD (8Tx-8Rx).
A typical approach to extending (E)PDCCH coverage is to use longer transmission time (such as using repetition or spreading for example) in order to allow the UE (mobile device) or eNB (base station) to correctly demodulate and decode the signal. To reach the target coverage improvement, a repetition factor of approximately 100 may use 8 control channel elements (CCEs) for LTE devices with 2 received antennas deployed in system bandwidth of 10 MHz (such as when the (E)PDCCH is transmitted over 100 ms). For low-cost MTC devices, which may have a reduced bandwidth capability (such as, 1.4 or 3 MHz), additional frequency diversity loss of 1-2 dB can be experienced. When a single receive RF chain is present, then a further 3-4 dB loss can be experienced. Thus, for low-cost MTC, a repetition factor in the order of 400-800 times may be needed for cell-edge UEs.
There is considerable overhead to support all these repetitions. For instance, using aggregation level 8 for PDCCH and 100 repetitions for 10 MHz, 800 CCEs will be taken by a single device for a single transmission. This may be seen as 400 missed scheduling opportunities (assuming an average CCE aggregation level of 2). This overhead may be significantly worse for low-cost MTC devices. Further, if a reduced bandwidth of 1.4 MHz is used and the device has only one receiver chain, then 3,200 CCEs may be taken up. This may be seen as 1600 missed scheduling opportunities or being able to only schedule one device in the downlink in 1 second.
What is needed are techniques to reduce the overhead while providing coverage enhancement for machine type communications.